The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus having digital optics for forming a latent image on an image carrier and, more particularly, to an image forming apparatus of the type feeding a bias current smaller than a predetermined value to a light source included in digital optics even when image formation is not under way.
An image forming apparatus of the type uniformly charging a photoconductive element or similar image carrier and then scanning it with digital optics including a light source in response to image data is conventional. The light source is implemented as a laser or similar light emitting diode (LED). Generally, image data are written on the image carrier in high dot density and in dots each having multiple levels in order to attain high resolution and high tonality at the same time. To provide a dot with multiple levels, the laser is usually modulated by power modulation (PM) or pulse width modulation (PWM) which respectively control the intensity and the duration of light to issue from the laser. While an image forming apparatus of the type having digital optics has customarily been implemented as a terminal associated with a computer, it is increasingly applied even to ordinary copiers due to the recent remarkable cost reduction of semiconductor parts. Regarding a computer, it is not necessary for this type of apparatus to operate at high process speed. However, when it comes to an ordinary copier, the apparatus is required to implement high productivity and, therefore, high process speed, resulting in an increase in the frequency for modulating the laser.
Generally, to provide a laser beam with particular intensity, a particular drive current has to be fed to the laser, as understood from the relation between the drive current applied to the laser and the emission intensity. However, when the modulation frequency for modulating the laser 1 is increased, a drive current circuit and the laser itself cannot follow the modulation frequency due to responses particular thereto. This problem is especially serious with PWM which controls the duration of emission of the laser, as stated above. When the emission intensity of the laser is short of required one, the quantity of light to incident on a photoconductive element becomes short. As a result, the image density decreases in a negative-to-positive type image forming apparatus which deposits a toner in the exposed portions of a photoconductive element, or the background is contaminated in a positive-to-negative type apparatus which deposits a toner in unexposed portions.
To eliminate this problem, it has been proposed to constantly apply a particular bias current to the laser, so that the laser drive current may rise to a drive value from the value of the bias current instead of from zero, as disclosed in Japanese Patent Laid-Open Publication No. 113472/1991 by way of example. This compensates for the slow start-up of the laser beam. It is a common practice to set the bias current in a range which causes the emission of the laser to sparingly appear as light intensity. Usually, when the drive current exceeds a predetermined threshold current, it brings the laser into an emission state and and increases the beam intensity in proportion thereto. The beam from the laser exposes the photoconductive element. When this occurs while the photoconductive element is out of rotation, the beam causes the element to optically fatigue and changes the charging condition of the element, thereby adversely influencing the resulting image. Preferably, therefore, the bias current should be maintained smaller than the threshold. For this reason, it has been customary to compare a laser output sensed by a photosensor and a target value and control a laser driver to equalize them.
However, to realize rapid response by increasing the process speed and dot density, it is necessary that a bias current close to the threshold current be selected. Then, in an apparatus selecting such a bias current, the bias current causes the laser to emit a beam due to the irregularity in the sensitivity of a photosensor and the response of a control system. As a result, the photoconductive element optically fatigues to degrade image quality. Moreover, when the beam intensity is excessively high, the beam appears in the exposing section as a flare to blur the latent image. Another conventional image forming apparatus is provided with a predetermined bias current greater than the threshold. Even in this type of apparatus, a deviation of the bias current from the predetermined value would aggravate the fatigue of the photoconductive element and change the contrast of an image.
Moreover, with no regard to the purpose of the bias current or whether or not the light source is a semiconductor laser, it is likely that the emission intensity due to the bias current exceeds expected one on account of the irregularity in the sensitivity of the photosensor and the response particular to the control system. Then, the emission will correspondingly accelerate the fatigue of the photoconductive element while aggravating the flare. It follows that even an image forming apparatus having an ordinary LED in place of the laser is apt to suffer from the above-described occurrences. Specifically, assuming a negative-to-positive type image forming apparatus which provides a single dot with multiple levels, the bias current may be constantly fed to the light source so as to obtain an emission intensity associated with the background of an image by feeding a predetermined bias current. The laser may be replaced with an ordinary LED which uses the proportional relation between the voltage fed and the quantity of light in the natural emission state.